Screw gears also sometimes called crossed helical gears, are helical gears used in motion transmission between non-intersecting shafts. The helical gears used in parallel shafts have the same helix angle but in the opposite directions.
Screw gears are used for offset shafts. Whether the shafts are paralleled offset or skewed offset depends on the helix hands of the mating gears.
Shaft angles other than parallel, an arbitrary helix angle can be chosen, and, assuming no profile shift, the intersecting angle of the shafts is the sum of the respective helix angles. Helical gears have point contacts and have lower efficiency than parallel or perpendicular shaft gears and cannot transmit heavy loads.